Insect Visitation and Pollen Deposition in an Invaded Prairie Plant Community
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View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by DigitalCommons@University of Nebraska University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln USGS Northern Prairie Wildlife Research Center US Geological Survey 2006 Insect Visitation and Pollen Deposition in an Invaded Prairie Plant Community Diane L. Larson USGS Northern Prairie Wildlife Research Center, [email protected] Ronald A. Royer Minot State University Margaret R. Royer Minot State University Follow this and additional works at: https://digitalcommons.unl.edu/usgsnpwrc Part of the Other International and Area Studies Commons Larson, Diane L.; Royer, Ronald A.; and Royer, Margaret R., "Insect Visitation and Pollen Deposition in an Invaded Prairie Plant Community" (2006). USGS Northern Prairie Wildlife Research Center. 85. https://digitalcommons.unl.edu/usgsnpwrc/85 This Article is brought to you for free and open access by the US Geological Survey at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in USGS Northern Prairie Wildlife Research Center by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. BIOLOGICAL CONSERVATION 130 (2006) 148– 159 available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/biocon Insect visitation and pollen deposition in an invaded prairie plant community Diane L. Larsona,*, Ronald A. Royerb, Margaret R. Royerb aUSGS Northern Prairie Wildlife Research Center, 100 Ecology Building, 1987 Upper Buford Circle, St. Paul, MN 55108, United States bDivision of Science, Minot State University, Minot, ND, United States ARTICLE INFO ABSTRACT Article history: Invasive plants with large flowering displays have been shown to compete with native Received 15 July 2005 plants for pollinator services, often to the detriment of native plant fitness. In this study, Received in revised form we compare the pollinator communities and pollen deposited on stigmas of native plant 28 November 2005 species within and away from stands of the invasive alien plant, leafy spurge (Euphorbia Accepted 14 December 2005 esula) at a large natural area in North Dakota, USA. Specifically, we ask if infestation influ- Available online 26 January 2006 ences (1) visitation rates and taxonomic composition of visitors to native flowers, and (2) the amount of conspecific pollen, number of pollen species, and proportion of heterospe- Keywords: cific pollen on stigmas of native plants. We observed visits to selected native species during Biological invasion May and June 2000 and 2001. Stigmas were collected from a subsample of the flowers Euphorbia esula within these plots, squashed, and the pollen identified and counted under a light micro- Halictidae scope. Visitation varied between years and among species of native plants: infestation Mixed-grass prairie had mixed effects in 2000 but visitation, especially by halictids was always lower within Pollen on stigmas infestations in 2001. Despite differences in visitation between years, we found significantly less conspecific pollen on stigmas from infested plots in six of eight cases; we never found significantly more conspecific pollen on stigmas from within infestations. Our results emphasize the temporal variability in plant–pollinator relations and the added complexity imposed by an invasive species that will always make prediction of effects difficult. Nonetheless, the consistently lower conspecific pollen counts on native stigmas within infestations, regardless of visitation, suggest the likelihood of negative effects. Ó 2005 Elsevier Ltd. All rights reserved. 1. Introduction infestation could be enhanced (Feldman et al., 2004); however, if those pollinators preferentially visit the invasive species, Many alien plants form dense monospecific stands that visitation to, and therefore pollination of, native plants could flower profusely, and thus may introduce substantial be depressed (Rathcke, 1983). Because pollination systems amounts of alien pollen and nectar into the native communi- tend to be generalized, especially in the northern temperate ties they invade. The diffuse nature of pollination mutualisms zone of North America (Johnson and Steiner, 2000), pollina- (Palmer et al., 2003) and the multiple interspecific interactions tion of native plants also could be depressed if pollinators involved (Waser et al., 1996) make the outcome of such an in- that visit native plants deposit alien pollen on their stigmas, flux of resources highly uncertain. If an increase in resources and much of the native pollen is carried to aliens, rather than attracts a disproportionately larger number of pollinators to to conspecifics (improper pollen transfer (IPT); Waser, 1978; the area, pollination of native plants in the vicinity of the Rathcke, 1983). Alien pollen that makes its way to native plant * Corresponding author: Tel.: +1 612 625 9271; fax: +1 612 624 6777. E-mail address: [email protected] (D.L. Larson). 0006-3207/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.biocon.2005.12.009 BIOLOGICAL CONSERVATION 130 (2006) 148– 159 149 stigmas may affect reproduction by clogging or mechanically Vegetation at the park is dominated by native cool season blocking the stigma or style, by chemically interfering with grasses, primarily Pascopyrum smithii and Stipa spp. The south fertilization (allelopathy), or by producing hybrids (Brown unit of the park was established in 1947 and contains free- and Mitchell, 2001; McLernon et al., 1996; Waser, 1978). ranging native ungulates as well as a small herd of wild Indeed, evidence has begun to accumulate that suggests horses. More detail on plant species associations within the invasive plants with large flowering displays can compete park can be found in Larson et al. (2001). E. esula was first doc- with native plants for pollinator services, often to the detri- umented in the South Unit of TRNP in 1970 (unpublished park ment of native plant fitness. Effects have been documented documents). Infested and non-infested areas used in this in terms of both pollen quantity and quality (Brown and study had been so since at least 1996. Mitchell, 2001; Brown et al., 2002; Chittka and Schu¨ rkens, Euphorbia esula is a Eurasian perennial that invades mixed- 2001; Moragues and Traveset, 2005). What is not yet clear is grass prairie in rangeland and natural areas throughout the whether the change in pollination success is due to the northern Great Plains and causes extensive environmental change in the prevalence of exotic pollen, which is dispersed and economic damage (Bangsund et al., 1996, 1999; Trammell (at the expense of native pollen) by the same generalist polli- and Butler, 1995). Aerial photography in 1991 indicated nator community that existed prior to infestation, a change in approximately 1620 ha of Euphorbia in the South Unit of TRNP, the pollinator community that visits native plants to fewer or with the distribution closely aligned with watercourses and less efficient pollinators, a change in rates of visitation to drainages. Euphorbia flowers profusely during June in North flowers, or some combination of these. If plants compete for Dakota, bearing terminal umbels with sequentially maturing pollinator services, a new plant species that provides copious female and male flowers (cyathia); each ‘‘wave’’ of flowers resources will dramatically alter the competitive relation- produces 5–12 female followed by 11–20 male flowers and ships within the plant community, with less abundant spe- each stem may branch to produce as many as 16 umbels (Sel- cies being put at a distinct disadvantage (Palmer et al., leck et al., 1962). Each plant’s yellow flowers mature in succes- 2003), especially with respect to constant pollinators that spe- sion over a period of several weeks. Pollen remains viable for cialize on abundant and rewarding flowers (e.g., Gegear and up to 2 days on each flower and nectar is produced by the Laverty, 1998, 2004; Kunin, 1993). In contrast, if pollinators glands of the cyathium 3–8 days after inversion of the female compete for floral rewards, introduction of a super-abundant flower (Selleck et al., 1962), so a consistent source of pollen resource may mean that pollen and nectar are no longer lim- and nectar remains available for approximately 6 weeks at iting, and new or formerly out-competed insect taxa may en- our study sites. Capsules, which are explosively dehiscent, ter the system. These issues are of concern not only with mature in mid- to late-summer and contain up to three seeds. respect to potential effects on pollen-limited plants, but also in light of recent indications of declining pollinator popula- 2.2. Sampling design tions (Kearns et al., 1998; Spira, 2001). In this study, we compare the pollinator insect communi- Using data from a 1996 stratified-random survey of plants in ties and pollen delivered to stigmas of native plant species the South Unit in which >800 geo-referenced transects were within and >100 m away from stands of the invasive alien examined (methods for the survey can be found in Larson plant, leafy spurge (Euphorbia esula L.; Euphorbiaceae; hereaf- et al. (2001)), we identified areas within the South Unit that ter Euphorbia). Specifically, we ask (1) does visitation to native were likely to have one of several concurrently blooming na- flowers by pollinator insect taxa vary between infested and tive plants, either >100 m away from or within a Euphorbia non-infested sites, (2) does the amount of conspecific pollen infestation.